Stephens flew nearly all jet fighter aircraft of the 1940s and 1950s era from the F-80 Shooting Star to the F-106 Delta Dart.[3] He flight tested the YF-100A, YF-102, XF-104, X-5, X-1B, Navy F2H3, F4D, the F-94 series, F-86, F-89D, F-89H, and YF-101.[7] He served as test director for the F-104 program and performed the first wingtip fuel tank jettison test. On this flight, the fuel tanks slammed into the fuselage of the F-104 instead of falling free. Stephens made an emergency landing and saved the aircraft. Working with Lockheed engineers and other aviation consultants, he helped solve this and other problems with the F-104.[8]

On 1 May 1965, at Edwards Air Force Base, pilot Stephens and fire control officer Daniel Andre, established four world speed and altitude records in the YF-12A.[9] They averaged 2,070 miles per hour over a 17-kilometer straight away course, then held 80,257 feet to establish a world record for sustained horizontal flight.[7][10][11] These records stood until 1977 when they were surpassed by an SR-71.[4]

After retiring from the military, Stephens was the U.S. representative for the German jet engine firm, MTU.